[Heathkit] Completed rebuilding the IT-12 Signal tracer, a couple glitches

Sat Jun 27 18:37:43 EDT 2015

revision page for IT-12; refers to power xformer and is for earlier manuals; probably not applicable to phil's problem, but may help others.nevadabarry.com/images/revision.gif tnx,barrycarson city, nv 

     On Saturday, June 27, 2015 3:19 PM, Ian Wilson <ianmwilson73 at gmail.com> wrote:

 I would replace C8 (goes from plate of the second triode to
ground); perhaps this is open circuit. Likewise C9 (is
across the output transformer).

Also check that the input potentiometer has continuity to
ground.

If not one of these, perhaps there is some ground connection
that is being made through a rivet or or other path that has
gone high-impedance.

73, ian K3IMW

On Sat, Jun 27, 2015 at 2:17 PM, Phil <ko6bb1 at gmail.com> wrote:

> Hi All,
> First, when I received this unit, I fired it up with the dim-bulb-tester
> just to be on the safe side.  It fired up, first with a few "crackles" and
> some hum.  Turning the volume control up past about halfway, give or take
> and it would break into oscillation.  I figured that was probably due to
> flaky filter caps and didn't worry about it at the time as I figured I'd
> re-cap it anyway.  Also, the eye tube was quite dim, could see it ONLY if
> the room lights were off.  I hoped that was due to low B+ or related.
>
> Further examination showed that most of the resistors were out of
> tolerance on the high side, most of the higher values ones by as much as
> 50%.  NOT unusual for carbon resistors.
>
> So I decided that a complete rebuild was in order, not just a basic
> re-cap.  So I tore it completely down to a bare chassis, gathered all the
> resistors and capacitors together and started the rebuild just like it was
> an unopened kit.  Even the tube sockets were replaced with ceramic ones
> (with tube shield for the 12AX7) (except the 1629 eye tube socket).
>
> The parts that were reused was the speaker, output Xformer, power Xformer,
> all controls & switches and of course any hardware/binding posts etc.  ALL
> small electronic parts (caps, resistors,rect etc) were replaced.
>
> I was VERY careful to make sure that the wiring is exactly per the manual,
> doing it step by step and check-marking the manual. The original can type
> filter capacitor was replaced by a couple terminal strips and discrete
> capacitors.  The 50uF input cap was replaced by a 40uF one and the 50uF
> output filter cap (after the 10K resistor) was replaced by a 100uF unit
> (both rated 260V). This because they're what I had on hand in my new stock.
>
> The "Wired-in" permanent probe cord was replaced by a 1/4" jack in the
> same hole, this because I don't like "wired-in" probes and other dangling
> things.  The power cord was replaced by a polarized one, and a .01uF cap
> placed from the neutral lead to the chassis. I made sure the "hot lead"
> went to the power switch.  I chose this route rather than a 3 wire cord
> (original was 2 wire non-polarized).  NO other changes were made to the
> unit.
>
> OK, here's the "glitches" I mentioned.
> 1.  The eye tube is still dim and will need replacement (whew, 18.00 at
> Antique Electronics).  Voltages on it are all correct.
>
> 2.  It STILL oscillates when the volume is turned up about half way, give
> or take.  Doesn't sound any different in that regard than it did when it
> arrived.  Putting the 12AX7 tube shield on/off doesn't change it any.  It's
> high pitched, wife thought I was playing with crickets in here ;-)  I tried
> subbing a 12AT7 thinking the lower mu device might not oscillate, but it
> made little/no difference.
>
> 3.  Hum is now negligible until the volume is turned up moderately,
> indicating any hum pick-up is 'probably' due to the close proximity of the
> power Xformer to the front end circuits. The unit is rather tightly packed
> and there isn't as much spacing between things as I'd do it if I designed
> it (I've done a lot of home and pro audio work).
>
> I wonder if any other list members have experience with the IT-12 signal
> tracer?  This is the one in the VTVM sized cabinet, I'm NOT sure if the
> larger cabinet ones would have similar oscillation problems, even IF they
> used the same circuit. . .
>
> --
> 73 de Phil,  KO6BB
> http://www.qsl.net/ko6bb/  (Web Page)
>
> PRESENT RADIOS:
> Grundigs:    S-350 (~2006), G6 (2011) & S450DLX (2014).
> Icom:        R-75 With Cascaded 250 & 125Hz CW Filters. (~2005)
> Icom:        IC-7200 Transceiver (~2015).
> Radio Shack: DX-380 digital portable (~1990).
> Zenith:      Royal-7000 Transoceanic Portable (~1968).
>
> ACCESSORIES:  Homebrew LF-MF Pre-Amp, MFJ-993B HF Auto-Tuner.
>              Homebrew 8 Hz Audio Filter.
>
> ANTENNAS:  88 foot Long Ladder-line fed dipole at 35 feet AGL.
>            Active Mini-Whip at 36 Feet AGL for LF/MW.
>            37 foot "Low Noise Vertical" at 11 feet AGL for LF/MW/SW.
>
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>
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